Method for regulation offset of current detection signal in driving power transmission controller

ABSTRACT

An offset value setting processing, whether or not all wheel velocities of front wheels and rear wheels are 0 km/h is determined and whether or not a pressing amount of an accelerator pedal is 0% is determined. If it is determined that the vehicle velocity is 0 km/h and that the pressing amount of the accelerator pedal is 0%, a current detection signal voltage at that time is set to an offset voltage. Thus, the offset voltage value is reset and set again if such a condition is established. Consequently, the offset voltage value including the amount of an error due to a temperature drift is set up, thereby preventing drop in control accuracy due to that error.

TECHNICAL FIELD

The present invention relates to an offset adjusting method for acurrent detection signal in a drive power transmission controlapparatus.

BACKGROUND ART

As an example of the drive power transmission control apparatus forvehicles, there are types in which the drive power transmission amountis controlled by duty-controlling an electromagnet in an“electromagnetic clutch” as disclosed in Japanese Utility Model No.HEI6-16731 or “drive power transmission control apparatus” as disclosedin an applied specification of JP No. 2001-003937 A. In such a drivepower transmission control apparatus, magnetic force for attracting anarmature to the electromagnet is generated by supplying excitationcurrent duty-controlled to an electromagnetic coil of the electromagnet,so that the armature is attracted to the side of a friction clutch.Consequently, the clutch is engaged with a pressure corresponding to themagnitude of the magnetic force so as to transmit the drive power. Thatis, torque to be transmitted to the front wheels and rear wheels of avehicle is divided depending on a current value of the excitationcurrent to be supplied to the electromagnetic coil of the electromagnet.

The excitation current to be supplied to the electromagnetic coil isdetected by a current detection circuit for detecting that excitationcurrent and inputted to the electronic control unit (ECU) of the drivepower transmission control apparatus as a current detection signal. As aresult, a control loop in which the input of the current detectionsignal is negative fed-back is constructed. Thus, the excitation currentfollowing a current instruction value determined by the ECU can besupplied to the electromagnetic coil of the electromagnet.

The current detection signal to be outputted from the current detectioncircuit is generally outputted through an operational amplifier after itis converted to a voltage signal. A configuration in which the voltagefrom an amplifying circuit is outputted as an offset voltage when nocurrent is supplied to the electromagnetic coil of the electromagnet,that is, the value of the excitation current is 0 A (zero ampere) isoften adopted. On the other hand, the output voltage by the currentdetection signal when the excitation current is 0 A is always deflecteddue to changes in the ambient temperature of the current detectioncircuit, deviation of the electric characteristic of component part andthe like.

Thus, when the value of the excitation current is 0 A, so-called zeropoint adjustment in which the output voltage of the current detectioncircuit at that time is set up as an offset voltage needs to be carriedout. Conventionally, a period in which no current is supplied to theelectromagnetic coil of the electromagnet at the time of startup isprovided and the offset adjustment control processing which performs thezero point adjustment in this while is carried out by the ECU which isthe drive power transmission control apparatus.

However, in the above-described offset adjustment processing, mostsemiconductor components, resistors, capacitors and the like of anoperational amplifier and the like constituting the current detectioncircuit have a particular temperature characteristic that theiramplification factor or impedance is changed due to changes in thetemperature. For the reason, so-called temperature drift that thevoltage of a detection signal is changed depending on changes in thetemperature of the current detection circuit may occur. Consequently,there is generated a deviation (Vof1-Vof2) in the offset voltage valuebetween the offset voltage value Vof1 (for example, ambient temperatureof 25° C.) at the time of offset adjustment processing executed justafter the startup and the offset voltage value Vof2 (for example,ambient temperature of 80° C.) just after a temperature rise. Becausethe deviation of the offset voltage value affects the voltage value of adetection signal from the current detection circuit directly, an erroroccurs in distribution of torque to be transmitted to the front wheeland rear wheel of the vehicle because of current control containing suchan error. As a result, there exists such a problem that a scheduledtorque distribution cannot be controlled precisely.

It may be considered depending on people that the not so remarkableerror in the current detection signal generated due to such atemperature drift hardly affects a feeling of passengers in the vehicleeven if there is generated the error in the distribution of the torquebetween the front wheel and rear wheel of the vehicle. However, theimprovement of the control accuracy in the vehicle drive powertransmission control apparatus is an indispensable matter because theimprovement of the control accuracy in the vehicle drive powertransmission control apparatus relating to such a factor contributes tosuppression of the deviation in the entire motion control system.

The present invention has been achieved to solve the above-describedproblem and an object of the present invention is to provide an offsetadjustment method for a current detection signal in the drive powertransmission control apparatus capable of controlling the torquedistribution precisely.

DISCLOSURE OF THE INVENTION

In order to achieve the above object, according to claim 1, an offsetadjusting method for a current detection signal in a drive powertransmission control apparatus for controlling a transmission amount ofdrive power by the a drive power transmission apparatus by controlling acurrent, is comprising:

-   -   a first step of determining whether or not all the wheel        velocities of vehicle wheels are 0 km/h;    -   a second step of determining whether or not the pressing amount        of the accelerator pedal is 0%; and    -   a third step of, if it is determined that all the wheel        velocities are 0 km/h in said first step and the pressing amount        of the accelerator pedal is 0% in said second step, the value of        said current detection signal at that time is set to an offset        value.

Further, according to claim 2, an offset adjusting method for a currentdetection signal in a drive power transmission control apparatus forcontrolling the transmission amount of a drive power by a drive powertransmission apparatus based on a current detection signal outputtedfrom a circuit for detecting a current, is comprising:

-   -   a first step of determining whether or not all the wheel        velocities of vehicle wheels are 0 km/h;    -   a second step of determining whether or not the opening degree        of a suction throttle valve is substantially 0%; and    -   a third step of if it is determined that all the wheel        velocities are 0 km/h in said first step and the opening degree        of the suction throttle valve is substantially 0% in said second        step, setting the value of said current detection signal at that        time to an offset value. “A suction throttle valve” means a        variable throttle provided halfway of an suction path of        internal combustion.

According to the inventions of claim 1 and claim 2, if it is determinedthat all wheel velocities are 0 km/h and that the pressing amount of theaccelerator pedal is 0% or the opening degree of a suction throttlevalve is substantially 0%, that is, if the vehicle is stopped or itsaccelerator pedal is not pressed by a vehicle driver, the value of thecurrent detection signal at that time is set to an offset value. Becausethe offset value is set when such a condition is established, thatoffset value can be reset and set again not only just after a startupbut also even when the ambient temperature of the current detectioncircuit is raised if the condition for the ambient temperature isestablished. Thus, even if the ambient temperature of the currentdetection circuit is raised, the offset value can be set taking thatmatter into account, thereby preventing generation of an error which mayoccur in a current detection signal due to a temperature drift.Therefore, control accuracy performance is improved, so that torquedistribution can be controlled at a high accuracy.

Further, according to claim 3, the offset adjusting method for a currentdetection signal in a drive power transmission control apparatusaccording to claim 1 or claim 2 wherein said third step is carried outafter a predetermined period passes since the value of said currentdetection signal is set to the offset value last.

Because according to the invention of claim 3, the third step isexecuted after a predetermined period passes since the value of thecurrent detection signal is set to the offset value last, setting of theoffset value in the third step is not executed even if the condition forthe first step and second step is established in the predeterminedperiod. Consequently, occurrence of such an unstable condition of theoffset value that the set offset value is reset and set again repeatedlyin a short period can be prevented. Therefore, torque distribution canbe controlled at a high precision under a stable control performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram showing the schematic structure of a4-wheel drive vehicle loaded with a drive power transmission controlapparatus according to an embodiment of the present invention;

FIG. 2 is a partial sectional view showing the structure of the drivepower transmission apparatus shown in FIG. 1;

FIG. 3 is a functional block diagram showing the outline of excitationcurrent control of an electromagnet by ECU 18 in the drive powertransmission control apparatus according to the embodiment; and

FIG. 4 is a flow chart showing the flow of the offset value settingprocessing by the ECU in the drive power transmission control apparatusaccording to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the embodiment of the offset adjustment method for acurrent detection signal in the drive power transmission controlapparatus of the present invention will be described with reference tothe accompanying drawings. For this embodiment, an example in which thepresent invention is applied to the drive power transmission controlapparatus for controlling the drive power transmission apparatus of a4-wheel drive vehicle will be described with reference to FIGS. 1-4.

FIG. 1 is an explanatory diagram showing the schematic structure of the4-wheel drive vehicle loaded with the drive power transmission controlapparatus 19 of this embodiment. FIG. 2 is a partial sectional viewshowing the structure of the drive power transmission apparatus 10 ofthis embodiment. Because the drive power transmission apparatus 10 issubstantially symmetrical with respect to a rotation axis L, it shouldbe noted that the same Figure represents substantially half section ofthe drive power transmission apparatus 10 while representation of theother half section is omitted.

As shown in FIG. 1, the drive power transmission control apparatus 19comprises the drive power transmission apparatus 10 and the ECU 18.Prior to description of the structures of the drive power transmissioncontrol apparatus 19 and the power transmission apparatus 10, thestructure of the 4-wheel drive vehicle loaded with the drive powertransmission apparatus 19 will be described with reference to FIG. 1.

In the 4-wheel drive vehicle, the trans axle 21 includes a transmission,transfer and front differential and outputs a drive power of an engine22 to both axle shafts 24 a, 24 a through a front differential 23 of thetrans axle 21 so as to drive the right and left front wheels 24 b, 24 b.Further, this drive power is outputted to the side of a first propellershaft 25 also.

The first propeller shaft 25 is connected to a second propeller shaft 26through a drive power transmission apparatus 10. If the first propellershaft 25 and the second propeller shaft 26 are connected to each otherso that a torque can be transmitted, a drive power of the engine 22 istransmitted to a rear differential 27 and outputted to both axle shafts28 a, 28 b from the rear differential 27 so as to drive the right andleft rear wheels 28 c, 28 d.

Rotation sensors 5, 6, 7, 8 for detecting the rotation velocity of thewheel are provided on vehicle wheels 24 c, 24 d, 28 c, 28 d and vehiclewheel velocity signals N1-N4 are outputted from the rotation sensors5-8. The respective vehicle wheel velocity signals N1-N4 are datacoinciding with or proportional to the rotation number (rpm) of eachwheel.

An indicated throttle valve provided halfway of an suction path of theengine 22 is provided with a throttle opening degree sensor 2 fordetecting the degree of opening of the throttle valve. A throttleopening degree signal m is outputted from the throttle opening degreesensor 2. Further, an indicated accelerator pedal is provided with anaccelerator pressing amount sensor 9 for detecting the pressing amountof the accelerator pedal. An accelerator pressing amount signal AC isoutputted from the accelerator pressing amount sensor 9.

The vehicle wheel velocity signals N1-N4, the throttle opening degreesignal m, the accelerator pressing amount signal AC, and the outputsignal of a drive mode selection switch 1 are inputted to the ECU 18.

The drive power transmission apparatus 10 is disposed between the firstpropeller shaft 25 and the second propeller shaft 26 and has a role oftransmitting and outputting a drive power inputted from the firstpropeller shaft 25 to the second propeller shaft 26. Here, the structureof the drive power transmission apparatus 10 will be described withreference to FIG. 2.

As shown in FIG. 2, the drive power transmission apparatus 10 comprisesan outer case 10 a, an inner shaft 10 b, a main clutch mechanism 10 c, apilot clutch mechanism 10 d, a cam mechanism 10 e and the like.

The outer case 10 a is comprised of a bottomed cylindrical housing 11 aand a rear cover 11 b which is fit to the rear end opening section ofthe housing 11 a through screw thread so as to cover the same openingsection. An end section of the first propeller shaft 25 shown in FIG. 1is connected to the front end of the housing 11 a which constitutes theouter case 10 a so that a torque can be transmitted between them.

The inner shaft 10 b is inserted into the outer case 10 a coaxially suchthat it goes liquid-tightly through the central section of the rearcover 11 b and supported rotatably by the housing 11 a and the rearcover 11 b in a condition that the motion thereof in the axial directionis restricted. Then, the front end of the second propeller shaft 26 isconnected to this inner shaft 10 b so as to be capable of transmittingthe torque.

A main clutch mechanism 10 c is a wet multiple-plate type frictionclutch, which comprises plural clutch plates of inner clutch plate 12 aand outer clutch plate 12 b, and disposed within the housing 11 a. Eachinner clutch plate 12 a is spline-fit to the outer periphery of theinner shaft 10 b so that it is capable of moving in the axial direction.Each outer clutch plate 12 b is spline-fit to the inner periphery of thehousing 11 a so that it is capable of moving in the axial direction. Theinner clutch plate 12 a and the outer clutch plate 12 b are disposedalternately and they can contact each other so as to engage withfriction and separate from each other to a free state.

A pilot clutch mechanism 10 d is an electromagnetic clutch, whichcomprises an electromagnet 13, a friction clutch 14, an armature 15 anda yoke 16.

An annular electromagnet 13 is constructed of an electromagnetic coil 13a wound around the rotation axis L and fit in an annular concave section11 d in the rear cover 11 b through a predetermined gap such that it isfit to a yoke 16. The yoke 16 is fixed to the body side in a conditionthat it is supported rotatably by the outer periphery of the rear endsection of the rear cover 11 b.

The rear cover 11 b is comprised of an inner cylindrical section made ofmagnetic material whose section in the radius direction is substantiallyL-shaped, an outer cylindrical section made of substantially annularmagnetic material provided on the outer periphery of that innercylindrical section and shutdown member 11 c made of substantiallyannular non-magnetic material fixed between the inner cylindricalsection and the outer cylindrical section.

The friction clutch 14 is a wet multiple-plate friction clutch comprisedof multiple clutch plates of outer clutch plates 14 a and inner clutchplates 14 b. Each outer clutch plate 14 a is spline-fit to the innerperiphery of the housing 11 a so that it is capable of moving in theaxial direction. Each inner clutch plate 14 b is spline-fit to the outerperiphery of a first cam member 17 a constituting a cam mechanism 10 e,which will be described later, so that it is capable of moving in theaxial direction.

The annular armature 15 is spline-fit to the inner periphery of thehousing 11 a so that it is capable of moving in the axial direction anddisposed in front of the friction clutch 14 such that it opposes thefriction clutch 14.

In the pilot clutch mechanism 10 d having such a structure, ifexcitation current is supplied to the electromagnetic coil 13 a in orderto excite the electromagnet 13, a loop-like circulating magnetic path isformed such that circulating magnetic flux passes from the electromagnet13 to the yoke 16, the rear cover 11 b, the friction clutch 14, to thearmature 15. The excitation current supplied to the magnetic coil 13 aof the electromagnet 13 is controlled as described later to apredetermined current value set up by duty-control of the ECU 18.

Turning ON/OFF the excitation current to be supplied to theelectromagnetic coil 13 a of the electromagnet 13 is carried out byselecting operation of the drive mode selection switch 1 shown in FIG.1, so that three drive modes can be selected. The drive mode selectionswitch 1 is disposed in the vicinity of a driver's seat in the vehiclecompartment so that it can be operated easily by the driver. In themeantime, if the drive power transmission control unit 19 is constructedwith only a second drive mode (AUTO mode) which will be described later,it is possible to omit the drive mode selection switch 1.

The cam mechanism 10 e which is a converting mechanism is comprised of afirst cam member 17 a, a second cam member 17 b and a cam follower 17 c.The first cam member 17 a is fit to the outer periphery of the innershaft 10 b rotatably and supported rotatably by the rear cover 11 b. Aninner clutch plate 14 b of the friction clutch 14 is spline-fit to theouter periphery thereof.

The second cam member 17 b is spline-fit to the outer periphery of theinner shaft 10 b so that it is capable of rotating integrally anddisposed to oppose the rear side of the inner clutch plate 12 a of themain clutch mechanism 10 c. A ball-like cam follower 17 c is fit intocam grooves opposing each other in the first cam member 17 a and thesecond cam member 17 b.

In the drive power transmission apparatus 10 having such a structure, ifthe electromagnetic coil 13 a of the electromagnet 13 which constitutesthe pilot clutch mechanism 10 d is supplied with no electricity, thatis, no excitation current is supplied, no magnetic path is formed, sothat the friction clutch 14 is in non-engagement state and consequently,the pilot clutch mechanism 10 d is in non-operating condition. Then, thefirst cam member 17 a which constitutes the cam mechanism 10 e becomescapable of rotating integrally with the second cam member 17 b throughthe cam follower 17 c, so that the main clutch mechanism 10 c turns intothe non-operating condition. Consequently, the vehicle turns into afirst drive mode (2WD mode) which is 2-wheel drive.

If the excitation current is supplied to the electromagnetic coil 13 aof the electromagnet 13, a loop-like circulating magnetic path is formedstarting from the electromagnet 13 in the pilot clutch mechanism 10 d soas to generate a magnetic force and consequently, the electromagnet 13attracts the armature 15. Thus, the armature 15 presses the frictionclutch 14 and engages with friction so as to produce a torque.Consequently, the first cam member 17 a of the cam mechanism 10 e isconnected to the side of the outer case 10 a so that a relative rotationis generated with the second cam member 17 b. As a result, in the cammechanism 10 e, the cam follower 17 c generates a thrust force formoving the both cam members 17 a, 17 b in directions that separates themfrom each other.

Thus, the second cam member 17 b is pressed to the side of the mainclutch mechanism 10 c so that the main clutch mechanism 10 c is pressedby the deep wall of the housing 11 a and the second cam member 17 b andthereby the main clutch mechanism 10 c is engaged with frictiondepending on the frictional engagement force of the friction clutch 14.Consequently, torque transmission is established between the outer case10 a and the inner shaft 10 b so that the first propeller shaft 25 andthe second propeller shaft 26 turn into a second drive mode (AUTO mode)which is 4-wheel drive located between the non-connecting state and thelock state. Under this second drive mode, the drive power distributionratio between the front and rear wheels can be controlled within a rangefrom 100:0 to its locked state depending on the traveling condition ofthe vehicle.

Under the second drive mode, supply of the excitation current to theelectromagnetic coil 13 a of the electromagnet 13 is duty-controlleddepending on the traveling condition of the vehicle and road surfacecondition based on signals from various kinds of sensors including therespective rotation sensors 5-8, the throttle opening degree sensor 2,the accelerator pressing amount sensor 9 and the like so as to controlthe frictional engagement force (that is, transmission torque to therear wheel side) of the friction clutch 14.

If the excitation current to the electromagnetic coil 13 a of theelectromagnet 13 is raised to a predetermined lock current which is aconstant value, the attraction force of the electromagnet 13 to thearmature 15 is increased, so that the armature 15 is attracted stronglythereby the frictional engagement force of the friction clutch 14 beingincreased. Consequently, the relative rotation between the both cammembers 17 a and 17 b is increased. As a result, the cam follower 17 cincreases the pressing force to the second cam member 17 b, so that themain clutch mechanism 10 c turns into coupled condition. Thus, the firstpropeller shaft 25 and the second propeller shaft 26 turn into a thirddrive mode (LOCK mode) which is 4-wheel drive in a lock state.

Next, the structure of the ECU 18 and the excitation current control ofthe electromagnet 13 by the ECU 18 will be described with reference toFIG. 3.

The ECU 18 comprises a CPU, memory, I/O interface, A/D converter, outputdrive circuit 18 f, current detection circuit 18 h (not shown) and thelike and enables control loop processing operation shown in FIG. 3 andthe like to be executed according to a predetermined control programstored in the memory.

That is, if vehicle velocity signals N1-N4 or an accelerator pressingamount signal AC are inputted into the CPU through the A/D converter orI/O interface (not shown), an instruction torque is generated by aninstruction torque generating section 18 a according to a predeterminedalgorithm or mapping processing using these signal data. A processingfor converting the instruction torque generated by this instructiontorque generating section 18 a to a torque current is carried out by atorque current converting section 18 b. Consequently, a currentinstruction value for generating an object torque is generated and thus,a differential between this current instruction value and a currentdetection signal Icp detected by the current detecting circuit 18 h isobtained by arithmetic operation. This differential is inputted to a PIcontrol section 18 d, in which proportional integration control iscarried out so as to compute an actually necessary excitation current.

Pulse width modulation is carried out by a PWM output converting section18e and a switching device Q is switching-controlling through an outputdrive circuit 18 f, so that excitation current is supplied to theelectromagnetic coil 13 a of the electromagnet 13 connected in seriesbetween the switching device Q and a battery B. Consequently, theloop-like circulating magnetic path is formed in the pilot clutchmechanism 10 d starting from the electromagnet 13 so as to generatemagnetism thereby the electromagnet attracting the armature 15. As aresult, the electromagnetic clutch of the pilot clutch mechanism 10 d isactivated.

On the other hand, the excitation current supplied to theelectromagnetic coil 13 a is detected by the current detecting circuit18 h. Because the excitation current supplied to the electromagneticcoil 13 a is converted to a voltage by a current detecting resistorlocated between the battery B and the electromagnetic coil 13 a, thecurrent detecting circuit 18 h is constituted of mainly an operationalamplifier so that a current detection signal voltage Vi can be outputtedwhen voltages generated on both ends of the current detecting resistoris inputted into the operational amplifier.

If a current detection signal voltage Vi outputted by this currentdetecting circuit 18 h is inputted to CPU through the A/D converter andthe I/O interface (not shown), a current detecting section 18 i computesa current detection signal Icp detected according to “linear approximateexpression or map of current detection signal voltage Vi and detectioncurrent” set up based on an offset voltage Vof set appropriately asdescribed later and this is inputted to an adder section 18 c through afilter section 18 j for removing unnecessary noise components.Consequently, the differential between the current instruction value bythe torque current converting section 18 b and the current detectionsignal Icp is computed by the adder section 18 c.

Under the excitation current control on the electromagnet 13 by the ECU18, the current detection signal voltage Vi detected by the currentdetecting circuit 18 h is converted to the current detection signal Icpwith reference to the offset voltage Vof. As described in theDescription of the Related Art, as the output voltage from theoperational amplifier, the offset voltage is outputted by theoperational amplifier constituting the current detecting circuit 18 heven when no excitation current is supplied to the electromagnetic coil13 a (excitation current 0 A). With this voltage as the offset voltageVof, the current detection signal voltage Vi is converted to the currentdetection signal Icp with a condition that that offset voltage Vof isadded.

Thus, if this offset voltage Vof suffers from a voltage deflection or atemperature drift occurs due to changes in the ambient temperature ofthe current detecting circuit 18 h which are generated because of thetemperature characteristic of the operational amplifier, resistor andcapacitor which constitute the current detecting circuit 18 h, it comesthat the current detection signal voltage Vi is converted to the currentdetection signal Icp with reference to the offset voltage Vof containingsuch a voltage deflection amount. Thus, the current detection signal Icpafter the conversion comes to contain errors due to the voltagedeflection.

According to this embodiment, the offset value setting processing shownin FIG. 4 is executed by the ECU 18. Here, the offset value settingprocessing will be described with reference to FIG. 4. This offset valuesetting processing is executed repeatedly and periodically (for example,every 5 milliseconds) by a predetermined timer interruption processingor the like.

As shown in FIG. 4, as the offset value setting processing, a processingof determining whether or not the 4-wheel velocity is 0 km/h is carriedout in step S101 after a predetermined initialization processing. Thatis, whether or not the velocities of the front wheel 24 b and the rearwheel 28 b which can be driven with a drive power transmitted by thedrive power transmission apparatus 10 are 0 km/h, that is, whether ornot the vehicle is stopped is determined.

If it cannot be determined that the 4-wheel velocity is 0 km/h in stepS101 (No in S101), the processing proceeds to step S103, in which thefirst timer value is cleared. This first timer value counts a time inwhich both the condition that “the 4-wheel velocity is 0 km/h” in stepS101 and the condition that “the accelerator pressing amount is 0%” instep S105, which will be explained next, are satisfied.

If it can be determined that the 4-wheel velocity is 0 km/h by thedetermination processing of step S101 (Yes in S101), the processingproceeds to step S105, in which whether or not the accelerator pressingamount is 0%. That is whether the accelerator pressing amount is 0% orthe accelerator pedal is pressed by a vehicle driver is determined basedon the accelerator pressing amount signal AC detected by theaforementioned accelerator pressing amount sensor 9.

If it can be determined that the accelerator pressing amount is 0% instep S105 (Yes in S105), the vehicle is stopped or its engine is stoppedor idled. Thus, a processing of incrementing the first timer value andthe second timer value in next step S107 is carried out. This secondtimer value is for counting a time since the previously set time in stepS113 and prevents the offset voltage value Vof from being set up againwithin a short period since the previous setting. It is requestedthrough the predetermined constant Tb that it is not set up again withinthe short period (for example, 10 seconds).

If it cannot be determined that the accelerator pressing amount is 0% bythe determination processing of step S105 (No in S105), the enginerotation speed is over the idling state even if the vehicle is stopped.Thus, the processing proceeds to step S103, in which the first timervalue is cleared.

If a processing of incrementing the first timer value in step S107 isended, a processing of determining whether or not the first timer valueis over a predetermined constant Ta is carried out in next step S109.This predetermined constant Ta sets up a time-up time of the first timervalue and for example, is set up to 100 milliseconds. Consequently, evenif both the condition that “the 4-wheel velocity is 0 km/h” in step S101and the condition that “the accelerator pressing amount is 0%” in stepS105 are incidentally satisfied by external noise or the like, theprocessing proceeds to step S117 if the first timer value is not overthe predetermined constant Ta (No in S109). Thus, an event that theoffset voltage value Vof is accidentally changed can be prevented.

That is, whether or not the first timer value has reached thepreliminarily set predetermined constant Ta is determined and if itcannot be determined that the first timer value has not yet reached thepredetermined constant value Ta (No in S109), the processing proceeds tostep S117, in which a processing of incrementing the second timer valueis carried out. On the other hand, if it can be determined that thefirst timer value has reached the predetermined constant Ta in step S109(Yes in S109), the processing proceeds to step S111, in which whether ornot the second timer value is over a predetermined constant Tb, orwhether or not the second timer value has reached the predeterminedconstant Tb is determined.

If it cannot be determined that the second timer value has reached thepredetermined constant Tb in step S111 (No in S111), a specific time(for example, 10 seconds) has not passed since the previous offsetvoltage Vof is set up. Thus, the processing proceeds to step S117, inwhich a processing of incrementing the second timer value is carriedout. On the other hand, if it can be determined that the second timervalue has reached the predetermined constant Tb in step S111 (Yes inS111), the processing proceeds to next step S112. In step S112, the PWMoutput is set to 0% duty, so that the real current is 0 A and then, theprocessing proceeds to step S113.

A processing for setting the current signal detection signal Vi at acurrent time (at that time) to the offset voltage value Vof is executed.That is, a case where step S113 is executed is the case where both thecondition that “the 4-wheel velocity is 0 km/h” in step S101 and thecondition that “the accelerator pressing amount is 0%” in step S105 aresatisfied. Because the transmission of drive power to the rear wheel is0(zero)N·m, which is an excellent condition, the PWM output is set to 0%duty and the switching device Q is turned OFF, so that the real currentis set to 0 A. The current signal detection voltage Vi of the currentdetecting circuit 18 h in this condition is set to the offset voltagevalue Vof.

When the vehicle is stopped and at the same time, the vehicle driverdoes not press the accelerator pedal, for example, the engine is stoppedor idled, the real current is 0 A and the current signal detectionvoltage Vi at this time is set to the offset voltage value Vof.Consequently, because the offset voltage value Vof is set up when such acondition is established, that offset voltage value Vof can be reset notonly just after a startup but also when this condition is establishedeven if the ambient temperature of the current detecting circuit 18 h israised. Because even if the ambient temperature of the current detectingcircuit 18 h is raised, the offset voltage value Vof can be set up basedon that fact, generation of an error which can be generated in thecurrent detection signal Icp by the temperature drift can be suppressed.

If the offset voltage value Vof is reset in step S113, the processingproceeds to step S115, in which a processing of clearing the secondtimer value is carried out and then, a series of the offset valuesetting processing is terminated. Consequently, the second timer valuewhich counts a time passing since a previous setting is initialized.Thus, the offset voltage value Vof is reset next time when the condition“the 4-wheel velocity is 0 km/h” in step S101 and the condition that“the accelerator pressing amount is 0%” in step S105 are satisfied afterthe second timer value reaches the predetermined constant Tb.

Although whether or not the accelerator pedal pressing amount is 0% isdetermined in the offset value setting processing explained withreference to FIG. 4, it is permissible to execute a processing fordetermining whether or not the opening degree of the throttle valve issubstantially 0% instead. That is, a throttle opening degree signal m isread through the throttle valve opening degree sensor 2 instead of theaccelerator pressing amount sensor 9 shown in FIG. 1 so as to determinewhether or not the opening degree of the throttle valve is substantially0%, that is, whether or not the accelerator pedal is pressed by avehicle driver. If it can be determined that the opening degree of thethrottle valve is substantially 0% in step S105 (Yes in S105), it meansthat the vehicle is stopped or its engine is stopped or idled. Thus, theprocessing proceeds to next step S107 and if such determination isimpossible (No in S105), it means that the engine is being driven beyondits idling state even if the vehicle is stopped. Thus, the processingproceeds to step S103. As described above, the same operation and effectcan be exerted even if the content of the processing by step S105 isreplaced.

According to the offset value setting processing by the ECU 18 of thedrive power transmission control apparatus 19 of this embodiment, asdescribed above, whether or not the wheel velocities of the front wheels24 c, 24 d and the rear wheels 28 c, 28 d are 0 km/h is determined bythe unit 19 in step S101, and whether or not the accelerator pedalpressing amount is 0% is determined in step S105. If it is determinedthat all the wheel velocities of the front wheels 24 c, 24 d and therear wheels 28 c, 28 d are 0 km/h in step S101 and simultaneously, it isdetermined that the accelerator pedal pressing amount is 0% in stepS105, the current signal detection voltage Vi at that time is set to theoffset voltage Vof in step S113.

That is, if the vehicle is stopped and simultaneously the acceleratorpedal is not pressed by a vehicle driver, for example, the engine isstopped or idled, the current signal detection signal Vi at that time isset to the offset voltage value Vof. Because the offset voltage valueVof is set up when there conditions are established, that offset voltagevalue Vof can be reset and set up again if those conditions areavailable for the ambient temperature even if the ambient temperature ofthe current detecting circuit 18h is raised. Thus, because the offsetvoltage value Vof can be set up by taking into account a rise in theambient temperature of the current detecting circuit 18 h even if thattemperature is raised, it is possible to suppress generation of adifference which may occur in the current detection signal Icp due tothe temperature drift. Therefore, because the control accuracyperformance can be improved by executing the differential computationusing the current detection signal Icp through the adder section 18 c,the torque distribution can be controlled at a high precision.

Because according to the offset value setting processing by the ECU 18of the drive power transmission control apparatus 19 of this embodiment,step S113 is executed if the second timer value exceeds thepredetermined value Tb (Yes in step S111), the setting of the offsetvoltage value Vof in step S113 is never executed even if the conditionsfor step S101 and step S105 are established until the second timer valuereaches the predetermined value Tb. Consequently, generation of such anunstable condition of the offset value Vof that the set offset value Vofis reset repeatedly in a short period can be prevented. Therefore, thetorque distribution can be controlled at a high precision under a stablecontrol performance.

1-3. (canceled)
 4. An offset adjusting method for a current detectionsignal in a drive power transmission control apparatus for controlling atransmission amount of drive power by a drive power transmissionapparatus by controlling a current, comprising: first determiningwhether or not all wheel velocities of vehicle wheels are 0 km/h; seconddetermining whether or not a pressing amount of an accelerator pedal is0%; and setting, if it is determined that all the wheel velocities are 0km/h in the first determining and the pressing amount of the acceleratorpedal is 0% in the second determining, a value of the current detectionsignal at that time to an offset value.
 5. The offset adjusting methodfor a current detection signal in a drive power transmission controlapparatus according to claim 4, wherein the setting is carried out aftera predetermined period passes since the value of the current detectionsignal is set to the offset value last.
 6. An offset adjusting methodfor a current detection signal in a drive power transmission controlapparatus for controlling a transmission amount of a drive power by adrive power transmission apparatus based on a current detection signaloutput from a circuit for detecting a current, comprising: firstdetermining whether or not all wheel velocities of vehicle wheels are 0km/h; second determining whether or not an opening degree of a suctionthrottle valve is substantially 0%; and setting, if it is determinedthat all the wheel velocities are 0 km/h in the first determining andthe opening degree of the suction throttle valve is substantially 0% inthe second determining, a value of the current detection signal at thattime to an offset value.
 7. The offset adjusting method for a currentdetection signal in a drive power transmission control apparatusaccording to claim 6, wherein the setting is carried out after apredetermined period passes since the value of the current detectionsignal is set to the offset value last.